Method of positioning an electro-optic probe of an apparatus for the measurement of voltage

Abstract: A method of positioning an E-O probe applied to an apparatus for the measurement of voltage. In the first step, the relative position of the E-O probe against the magnifying optical system in the first condition of being focused on the base of the E-O probe, and the focal distance difference between the focal plane of the magnifying optical system in the second condition that the E-O probe is substantially out of the optical path for observation of the surface of the device and the focal plane in the first condition are stored. The relative position and the focal point difference are fixed in the apparatus for the measurement of voltage regardless of the device to be measured. Next, in the second step, the focus of the magnifying optical system is adjusted to the observation position of the surface of the device in the second condition, and then in the third step, the magnifying optical system, the E-O probe, and the probe stage are separated from the device by the focal point difference stored in the first step by the Z-axis stage, and in the fourth step, the E-O probe is moved to the relative position of the E-O probe stored in the first step relative to the magnifying optical system by the probe stage.

Description: The invention relates to a method of positioning an apparatus comprising an objective lens and an electrooptic probe at a sample to be measured and to an electrooptic measuring apparatus. One possible feature of an apparatus for the measurement of voltage is an electrooptic probe (E-O) which comprises an electro-optic crystal the refractive index of which is varied when the voltage is applied. If the voltage is applied to a device to be measured while the E-O probe is close to the device, an electric field in the electro-optic crystal is varied, whereby the refractive index of the crystal is varied. At this point, if light (laser beam) enters into the E-O probe, the state of the light polarization is varied by a change of the refractive index of the crystal in accordance with the applied voltage. Accordingly, if the light which is reflected by a base of the probe and comes back to is taken out by a polarization beam splitter through a wave plate, a change of the applied voltage can be detected as a change of the light intensity. Here, a laser source for the measurement may be a continuous wave (CW) source or a pulsed source. The detecting sensitivity of the apparatus for the measurement of the voltage depends on the distance between the E-O probe and the device to be measured. Accordingly, it is important to set the distance accurately. A solution in a conventional case is, for example, to use a double-focus lens disclosed in the following reference "J.Appl.Phys.66(9), 1 November 1989 pp. 4001-4009", or to use a balance disclosed in the following reference "IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 41, NO. 3, JUNE 1992, pp. 375-380". However, in the former conventional case, an E-O probe can be positioned without contact with the device to be measured, but there is one fault that a special lens which is a double-focus lens is required. Further, in the latter conventional case, in order to set the position of the E-O probe, the E-O probe is brought into contact with the device to be measured once to obtain the reference position, and based on the reference position, the position of the E-O probe is determined. In this case, its structure is very complicated to decrease the damage due to the contact. Specifically, the E-O probe is needed to be fixed at the cylinder which can move up and down in the state of small friction with air blows between a cylinder and an air guide, and their effective mass is reduced by the balance system. Moreover, the slight vertical movement of the E-O probe is needed to be operated by a piezo element fixed at the cylinder, and the position of the E-O probe is needed to be read by a position scale installed at the cylinder. Thus. in the latter case. since the peripheral systems of the E-O probe were very complicated and large, the number of electrical pins, which provide voltage or signal could not be set around the E-O probe. Further, since the E-O probe is bought into contact with the device to be measured prior to the measurement, there is a danger of breaking the device. In the proceedings of 23rd International Test Conference 1992, Baltimore 20-24 Sept. 1992, paper 39.1, pp. 834-839 there is described an E-O probe positioner comprising an E-O probe tip, a probe holder an objective lens and a piezoelectric device. A weight balanced method for positioning the probe at a sample is also described. It is an aim of the present invention to provide a method of positioning an E-O probe of an apparatus for the measurement of voltage which does not require a special optical system and which can position the E-O probe without contact with a device to be measured. According to the present invention there is provided a method of positioning an apparatus comprising an objective lens and an electro-optic probe at a sample to be measured, the method comprising focusing the objective lens to determine the focal length of the objective lens; determining the combined focal length of the objective lens in combination with the probe when the base of the probe is at the focus of the combination; determining and storing the difference between the focal length of the objective lens and the combined focal length; placing the probe at a distance from the objective lens less than the combined focal length; placing a sample in position; focusing the objective lens on the sample; moving the objective lens away from the sample by a distance substantially equal to the determined focal length difference; and moving the probe so that the base thereof is positioned at a distance from the objective lens equal to the combined focal length. According to the present invention there is also provided an electro-optic measuring apparatus comprising an electro-optic probe; an objective lens means for moving the electro-optic probe relative to the lens in the direction of the optical axis of the lens; a pedestal for supporting a sample; means for effecting relative movement between the pedestal and the objective lens together with the electro-optic probe; means for focusing the objective lens to determine the focal length of the objective lens, the combined focal length of the objective lens in combination with the probe when the base of the probe is at the focus of the combination, and the difference between the focal length of the objective lens and the combined focal length, means for storing the determined difference in focal length, the apparatus being operable, with a sample supported on the pedestal, to position the probe at a distance from the objective lens less than the combined focal length, to focus the objective lens on the sample, to move the objective lens away from the sample by a distance substantially equal to the determined and stored focal length difference, and to move the probe so that the base thereof is positioned at a distance from the objective lens substantially equal to the combined focal length, whereby the base of the probe is positioned at the sample. The invention will become more fully understood from the detailed description of an embodiment given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the invention. Further scope of applicability of the invention will become apparent from the detailed description of the embodiments given hereinafter. However it should be understood that the detailed description and specific examples, while indicating presently preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description. In the drawings: Fig. 1 is a configuration of an apparatus for the measurement of voltage to which a method embodying the invention is applied. Fig. 2 is a view showing the measurement procedure according to a method embodying the invention. Fig. 3 is a view of the explaining main parts of Fig. 2. DETAILED DESCRIPTION OF THE EMBODIMENTS The embodiments of the present invention will be described hereunder. Fig. 1 shows the whole structure of an apparatus for the measurements of voltage to which a positioning method of the present embodiment is applied. As shown in Fig. 1, a pedestal 11 is placed on an upper surface of a holddown body part 1A and a device 12 to be measured is set on the part 1A. Further, an operation table 13 is fixed on the holddown body part 1A nearby the pedestal 11, and a manipulator 14 is installed on the operation table 13. Then, an electrical pin 15 is placed on the manipulator 14, which provides a power supply or a signal input to the device. An X-Y axes stage 21 is installed at a pillar part of the holddown body part 1A, and a moveable body part 1B is installed on the X-Y axes stage 21. A Z-axis stage 22 is installed at the moveable body part 1B. Then a microscope unit 3 is installed at a Z-axis stage 22, and a probe stage 4 is fixed at the microscope unit 3. A support pin 41 of tungsten extends from the probe stage 4, and the E-O probe 42 is supported by the end of the support pin 41. The microscope unit 3 comprises an objective lens 31 which faces a surface of the device 12 through the E-O probe 42. Further, the microscope unit 3 comprises a light source 32 for illuminating the surface of the device 12, a CCD camera 33 for taking an image of the surface of the device 12, and an eyepiece 36. Note that for the E-O probe 42 of ZnTe crystal, a visible-ray camera is used as the CCD camera 33, and for GaAs crystal, an infrared camera is used. The microscope unit 3 comprises dichroic mirrors 34 and 35 which couple the above mentioned light source and the imaging system, and a laser optical system for the voltage measurement which will be described later. That is, the dichroic mirror 34 transmits 50% of light of the wavelength from the light source 32 and reflects light from a laser beam source (described later). Further, the dichroic mirror 35 reflects light of the wavelength from the light source 32, and transmits light of the wavelength from the laser beam source. As described above, the objective lens 31 is used for both illuminating and taking the image of the device 12, and radiating laser beams for the voltage measurement. A voltage measuring unit 5 which is operated by a laser beam is installed at the microscope unit 3. Light coming out from the CW laser diode 51 goes to an optical isolator 53 through the lens 52. The optical isolator 53 transmits light only in one direction, and the transmitting light passes through a polarization beam splitter 54 and a 1/8-wave plate 55, and its light path is changed by mirrors 56 1 and 56 2 . Then, the light is reflected by the dichroic mirror 34, passes through the magnifying lens system 31, and goes to the E-O probe 42. The laser beams reflected by the bottom surface of the E-O probe 10 pass the magnifying lens system 31 again and 1/8-wave plate 55, then goes into polarization beam splitter 54. At this point, because the laser beams pass the 1/8-wave plate 55 twice, it is shifted by quarter-wave in phase, whereby the laser beams are circularly polarized. Accordingly, half of the incident laser beams to the polarization beam splitter 54 is reflected here and goes to the photodiode 57. Then, an output of the photodiode 57 is amplified by an amplifier 58 and is output to the outside. Here, the principle of the measurement of the voltage V in will be described briefly. If the voltage V in is applied to an electrode (not shown) on the surface of the device 12, the refractive index of the E-O probe 42 neighboring the electrode is changed by an electro-optic effect. Then, the laser beams (linearly polarized) are polarized while traveling through the crystal back and forth. Therefore, the ratio of the laser beam reflected by the polarization beam splitter 54 and going to the photodiode 57 is changed, so that the voltage V in of the device 12 can be measured from the output of the photodiode 57. The measurement results are displayed on a digital oscilloscope 61, and processed by a computer 62. Note that the monitoring image taken by a CCD camera 33 is displayed on a TV monitor 63. Here, the TV monitor 63 comprises memories for picture, so that the obtained image can be stored therein as a still picture (freezing image), and displayed thereon. Alternately, the monitoring image taken by the CCD camera 33 is entered and stored in the computer 62, and then displayed on a monitor of the computer 62, or on the TV monitor 63. The computer 62 controls a Z-axis stage 22 by using an automatic focusing device 64, and further controls the X-Y axes stage 21 and the probe stage 4. As described above, features of the apparatus to which the embodiments of the present invention are applied are that the microscope system including an objective lens for focusing the incident light to the E-O probe 42 has the automatic focusing function, and that the probe stage 4 for holding the E-O probe 42 is assembled to the microscope system, and that the external control device which moves the E-O probe 42 and the microscope system up and down by the procedure described hereinafter is installed. Here, the CCD camera 33 takes the image observed by the microscope system, and based on the results of the image processing obtained from the information such as contrast of the image, the automatic focusing device 64 adjusts the focus of the image by moving the microscope system up and down. Next, a positioning method of the present embodiment which is applied to the apparatus for the measurement of voltage as described above will be explained. Fig. 2 shows the measurement procedure and Fig. 3 shows main parts of the procedure. Referring to Fig. 3, the point will be explained briefly. First, as shown in (a) of Fig. 3, when the E-O probe 42 is substantially out of sight of the magnifying lens system 31 (there are a case of the E-O probe 42 close to the objective lens 31 as shown in (a) of Fig. 3 and a case that the E-O probe 42 is completely out of sight), the focal plane by the objective lens 31 is at F 1 . On the other hand, when the E-O probe 42 is in the optical path, as shown in (b) of Fig. 3, the focal plane by the objective lens 31 comes to F 2 , and a difference is made between F 1 and F 2 . This is because the refractive index of the E-O probe is generally larger than that of air, so that if the E-O probe 42 is in the optical path, the length of the optical path becomes long. Then, shown as a solid line in (c) of Fig. 3, the objective lens 31 is positioned against the device to be measured allowing the difference (F 1 - F 2 ) of the focal planes, and the E-O probe 42 shown as a dotted line in (c) of Fig. 3 is set, so that a laser beam for the measurement is focused on the base of the E-O probe 42. Referring to the measurement procedure as shown in Fig. 2, the method of positioning as described above will be explained in detail. Prior to the measurement, the initialization is required. (A) Initialization Initialization is to obtain the position H 0 of the probe stage 4 (more precisely, the amount of the downward movement of the E-O probe 42 by the probe stage 4) at the time of the measurement as shown in the operation item (4) of Fig. 2, to set the variation ΔH of the position of the probe stage 4 at the time of the preparation for the measurement (see operation item (1) of Fig. 2), and to obtain the focal point difference ΔZ of the objective lens 31 when the E-O probe 42 exists in the optical path and when the E-O probe 42 does not exist in the optical path (see the operation items (3)-(5) in Fig. 2). These values are set only once when the E-O probe 42 is replaced. Note that in the initialization, the device to be measured is not needed but the initialization is adjusted based on the image of the E-O probe 42 taken by the CCD camera 33, so that there is no danger of breaking the E-O probe 42. H 0 gives the amount of downward movement of the E-O probe 42 seen from the probe stage 4 at the time of measurement. H 0 is the position of the probe stage 4 when the focus of the objective lens 31 of the microscope is adjusted to the reflecting film on the base of the E-O probe 42 by moving the E-O probe 42 supported by the probe stage 4 in Z direction. Moreover, the lens 52 of the CW laser diode 51 is adjusted to make the smallest spot of the incident light to the E-O probe 42. ΔH is a distance that the E-O probe 42 is apart from the device 12 to be measured at the time of the preparation for the measurement. ΔH can arbitrarily be set, but ΔH is needed to be large enough so that the E-O probe 42 does not interrupt the sight of the microscope as shown in Fig. 2. ΔZ is the difference of the focal points of the microscope when the E-O probe 42 exists in the optical path and when the E-O probe 42 does not exist in the optical path. For example, a case of the E-O probe 42 made of ZnTe crystal and silica glass which is a support will be considered. In the case of ZnTe having the 2.9 refractive index and the 100 µm thickness and silica glass having the 1.5 refractive index and the 200 µm thickness, the practical optical path length L 1 is L 1 =2.9×100+1.5×200=590 µm. In the case of E-O probe 42 not existed in the optical path, since the refractive index of air is 1 and the space is 300 µm, the optical length L 2 is L 2 =300 µm. Accordingly, the focal point difference ΔZ in this case is ΔZ = L 1 - L 2 = 290 µm. ΔZ can be calculated if the structure of the E-O probe 42 and its size are known, but it can also be obtained by the following measurement. Instead of the device 12 to be measured, a sample for the ΔZ measurement is placed on the pedestal 11. The sample the contrast of which is large such as a stripe electrode in few µm width is appropriate therefore. First, the position of the probe stage 4 is set at H 0 -ΔH, and the focus of the objective lens 31 of the microscope is adjusted to the sample, and the position Z 1 at this time is stored. Next, the microscope is raised enough for a moment, and the position of the probe stage 4 is set at H 0 -a. At this time, a is at a level such that the E-O probe does not come into contact with the sample for the ΔZ measurement, for example, a few µm. In this condition, the focus of the objective lens 31 of the microscope is adjusted to the sample by looking the sample through the E-O probe, and the position Z 2 at this time is stored. The difference between Z 1 and Z 2 is the above-described focal point difference ΔZ. Here, while measuring Z 2 , the probe does not come into contact with the sample, but the focus of the objective lens 31 of the microscope is adjusted to the sample through the E-O probe, so that the condition is the same as (b) of Fig. 3. (B) Measurement procedure Referring to Fig. 2, the measurement procedure will be explained in order. (1) Movement of the measuring point In a case of moving the measuring point, the position of the probe stage 4 is set at H 0 -ΔH so that the E-O probe 42 does not come into contact with the device 12 to be measured. At this time, the focus of the microscope is adjusted by the automatic focusing function. Since the E-O probe 42 is separated from the device 12 at a distance of ΔH, there is no danger of contact with each other. In this condition, the measuring point is determined by moving the microscope or the device 12 in horizontal direction. (2) Determination of the measuring point Once the measuring point is determined, the focus of the device 12 is readjusted by the automatic focusing function in accordance with the instruction from the computer 62. That is, while the E-O probe 42 is substantially out of the optical path which observes the surface of the device 12, the focus of the microscope is adjusted. The image at this time is stored in the TV monitor 63 or in the computer 62. (3) Preparation for the measurement The Z-axis stage 22 is operated to raise the microscope system 3 by ΔZ. At this time, the image of the device 12 by the microscope on the TV monitor 63 is blurred and cannot be seen. (4) At measurement The E-O probe 42 descends by ΔH and the position of the E-O probe 42 is set at H 0 . At this time, the base of the E-O probe 42 comes into contact with the surface of the device 12. If the measurement is desired to be performed with the E-O probe 42 apart from the device 12 at a distance d, in the step of preparing for the measurement (3), the microscope system 3 may be raised by ΔZ + d. At this time, if the surface of the device 12 is observed on the TV monitor 63 though the E-O probe 42, since the device 12 is apart from the focal point at a distance d, the image of the device 12 is blurred and cannot be seen. As the countermeasure for this, the image stored in the step (2) of determining the measuring point is displayed on the TV monitor. (5) End of the measurement The probe stage 4 is operated to raise the E-O probe 42 and the E-O probe 42 is positioned back to H 0 - ΔH for next measurement. In the present embodiment, the position of the focal point of the microscope adjusted to the device 12 to be measured by using the automatic focusing function is used as the reference for determining the distance between the E-O probe 42 and the device 12 to be measured. In a case of a high magnification microscope, since the depth of field is narrow, the accuracy below 1 µm can easily be obtained. The E-O probe 42 is moved by the amount determined in the initialization, so that even though there is no sensor to detect the position of the probe, the E-O probe 42 can be placed near the device 12 at high accuracy. Thus, as described above, according to the method of positioning the E-O probe, the peripheral systems can easily be constructed. Further, when the position of the E-O probe and the amount of the movement of the microscope are determined in the initialization, the device to be measured is not needed to be set, so that there is no danger of breaking the device. From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims: A method of positioning an apparatus comprising an objective lens (31) and an electro-optic probe (42) at a sample (12) to be measured, the method comprising: focusing the objective lens (31) to determine the focal length of the objective lens (31); determining the combined focal length of the objective lens (31) in combination with the probe (42) when the base of the probe (42) is at the focus of the combination; determining and storing the difference between the focal length of the objective lens (31) and the combined focal length; placing the probe (42) at a distance from the objective lens (31) less than the combined focal length; placing a sample (12) in position; focusing the objective lens (31) on the sample; moving the objective lens (31) away from the sample (12) by a distance substantially equal to the determined focal length difference; and moving the probe (42) so that the base thereof is positioned at a distance from the objective lens (31) equal to the combined focal length. A method according to claim 1, further comprising moving the objective lens (31) away from the sample (12) by a predetermined further amount in addition to moving the objective lens (31) by the distance substantially equal to the determined focal length difference. A method as claimed in claim 1 or 2, further comprising displaying the image from the objective lens (31) on a monitor (63). A method as claimed in claim 1, 2 or 3, further comprising storing the image from the objective lens when the objective lens is focused on the sample (12). A method as claimed in any preceding claim, wherein the focal length difference is determined by calculation from the refractive index and the thickness of the electro-optic probe (42). A method as claimed in any of claims 1 to 4, wherein the focal length difference is determined by measuring the distance between the lens (31) and the sample (12) when the lens (31) is focused on the sample (12) and measuring the distance between the lens (31) and the base of the probe (42) when the lens (31) is focused on the base of the probe (42). An electro-optic measuring apparatus comprising: an electro-optic probe (42); an objective lens (31); means (4) for moving the electro-optic probe (42) relative to the lens (31) in the direction of the optical axis of the lens; a pedestal (11) for supporting a sample; means (22) for effecting relative movement between the pedestal and the objective lens (31) together with the electro-optic probe; means for focusing the objective lens (31) to determine the focal length of the objective lens (31), the combined focal length of the objective lens (31) in combination with the probe (42) when the base of the probe (42) is at the focus of the combination, and the difference between the focal length of the objective lens (31) and the combined focal length; means for storing the determined difference in focal length, the apparatus being operable, with a sample (12) supported on the pedestal (11), to position the probe (42) at a distance from the objective lens (31) less than the combined focal length to focus the objective lens (31) on the sample (12), to move the objective lens (31) away from the sample (12) by a distance substantially equal to the determined and stored focal length difference, and to move the probe (42) so that the base thereof is positioned at a distance from the objective lens (31) substantially equal to the combined focal length, whereby the base of the probe (42) is positioned at the sample (12). An apparatus as claimed in claim 7, wherein the objective lens (31) is movable away from the sample (12) by a predetermined further amount in addition to being movable by the distance substantially equal to the predetermined focal length difference. An apparatus as claimed in claim 7 or 8, further comprising a monitor (63, 62) for displaying the image from the objective lens. An apparatus as claimed in claim 7, 8 or 9, further comprising a store (63, 62) for storing the image from the objective lens (31).

Inventors: AOSHIMA SHINICHIRO, HIRANO ISUKE, TAKAHASHI HIRONORI, TSUCHIYA YUTAKA, AOSHIMA, SHINICHIRO, HIRANO, ISUKE, TAKAHASHI, HIRONORI, TSUCHIYA, YUTAKA, AOSHIMA, SHINICHIRO, C/O HAMAMATSU PHOTONICS K.K., HIRANO, ISUKE, C/O HAMAMATSU PHOTONICS K.K., TAKAHASHI, HIRONORI, C/O HAMAMATSU PHOTONICS K.K., TSUCHIYA, YUTAKA, C/O HAMAMATSU PHOTONICS K.K.
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1994
Main classification: G02B27
IPC codes: G01R31, G01R19